Painless has a strict whitelist for methods and
classes to make sure that all painless scripts are secure and fast. Most of
these methods are exposed directly from the JRE while others are part of
Elasticsearch or Painless itself. Below is a list of all available methods
grouped under the classes on which you can call them. Clicking on the method
name takes you to the documentation for the method.

Methods defined in the JRE also have a (java 9) link which can be used
to see the method’s documentation in Java 9 while clicking on the method’s name
goes to the Java 8 documentation. Usually these aren’t different but it is
worth going to the version that matches the version of Java you are using to
run Elasticsearch just in case.